For the first time, a federal appeals court has ruled that prosecutors cannot force a suspect to unscramble his encrypted computer hard drives. The Atlanta-based appeals court says that would violate the man's Fifth Amendment right against self incrimination.

NPR justice correspondent Carrie Johnson reports that, given the popularity of encryption software, the ruling could have big implications for other investigations.

CARRIE JOHNSON, BYLINE: Nearly two years ago, investigators tracked a man they called John Doe to a hotel in California and applied for a warrant to search his room. Inside, they found two laptops and five computer hard drives they believe contain images of underage girls. But when the FBI analyzed the devices, they couldn't see as many as 25 million files. Those files were locked using a common encryption program that's free on the Internet.

Lawyers for John Doe argued that turning over the password would violate his Fifth Amendment rights, essentially making him testify against himself. And this week, a federal appeals court based in Atlanta agreed.

Orin Kerr teaches computer crime at George Washington University.

PROFESSOR ORIN KERR: If other courts agree with this holding, it's going to significantly shift the power of the government versus the power of individuals in computer crime investigations.

JOHNSON: The John Doe case isn't really about access to the hidden computer files themselves. Instead, Kerr says, the issue is what the suspect would say through the fact of handing them over.

KERR: The idea is when a person discloses the document, they are implicitly saying this document is mine, I control it and it's real.

JOHNSON: And those questions go to the heart of what's in a suspect's head. Marcia Hoffman is an attorney at the Electronic Frontier Foundation in San Francisco.

MARCIA HOFFMAN: If you are forced to turn over a key to a lock box, you know, that really doesn't reveal much about the contents of your mind. But if you're asked to turn over the combination to a combination lock, then that does reveal the contents of your mind.

JOHNSON: The ruling in the John Doe case conflicts with two lower court rulings, where judges in Colorado and Vermont have ordered suspects to unscramble their hard drives. Hoffman says the new case involves a critical difference.

HOFFMAN: The fact that the government didn't know what they expected to find on his laptop. They wanted to go through his decrypted information in order to find evidence of a crime, but it was basically a fishing expedition.

JOHNSON: Not so in the Vermont case. There, an immigration agent found pornography on a computer during a border search, but he couldn't get at those images later because they had been encrypted. The Vermont judge decided that turning over the decryption password was just a foregone conclusion; a formality to information the government already knew.

The John Doe decision is the first by a higher level federal appeals court, but legal experts say it won't be the last. Encryption software now comes built in on many computers and some companies require employees who travel with laptops to use it.

Orin Kerr says that cuts both ways.

KERR: I think the fact that the press is reporting on these cases and that people are interested in these cases, it's going to alert individuals, hey, wait a minute, this is actually a tool that people can use to protect their privacy for good reasons and also for bad ones.

JOHNSON: And the novelty of encryption is also giving rise to a defense that might be familiar to teachers and parents everywhere. John Doe and a defendant in the Colorado case have both argued they don't know how to unscramble their hard drives.